1. Field of the Invention
The invention relates to a phase locked loop (PLL) and, in particular, to a mixed-mode PLL insensitive to quantization error.
2. Description of the Related Art
A digitally controlled oscillator-based (DCO-based) phase locked loop (PLL) uses a scalable digital loop filter which not only eliminates noise induced by gate oxide leakage but also digitally calibrates output frequency through a digital loop filter. However, this PLL is still limited to integer-N operations, because quantization errors of a digital PFD increases noise and spur in fractional-N operations.
Quantization errors of a time to digital converter (TDC) can not be shaped to a higher frequency domain such that the loop bandwidth is required to be set narrower, to suppress quantization errors and out-of-band noises. However, the in-band noises induced by the TDC still can not be filtered by the loop. Moreover, less DCO noise is filtered by the loop due to the narrower loop bandwidth, and in-band phase noise is thus consequently increased.
Non-linearity of the loop includes loop gain variation and mismatch for positive and negative phase error induced by quantization errors and meta-stabilities of the TDC. Gain mismatch and variation, induce fractional spurs when a high order of SDM is used to eliminate idle tone of fractional channels. A digital loop filter or a digital algorithm cannot eliminate the induced spurs since the quantization itself can not be cancelled by only using a digital algorithm.
Quantization errors of a DCO cannot be filtered by the loop because the noise transfer function of the DCO is a high pass filter, even when a high speed sigma delta modulator is used to obtain fine frequency resolution. The short-term quantization errors of the DCO is sampled back to the loop and amplified due to gain variation of the TDC, resulting in spurs. Meanwhile, a digitally-implemented ring type TDC must be used to achieve both large dynamic range and fine resolution for the delta-sigma fractional-N operation. The digitally-implemented ring type TDC is more sensitive to the power supply due to the ring based design, because the high frequency noises induced by power noise of TDC are aliased to a low frequency domain, which cannot be filtered by the loop.